The present invention relates to vacuum microelectronics and particularly to micro-patterned, microtip emitter structures made from diamond and similar materials, such as field emitter elements/arrays for use as vacuum diodes, triodes, sensors, displays, and other related applications.
The advance in integrated circuit fabrication and silicon micromachining technology has given an impetus to the development of vacuum microelectronic devices. Central to the field of vacuum microelectronics is the search for a high efficiency electron emission cathode. In recent years, many different materials, structures, and techniques have been investigated for fabrication of vacuum cold cathode devices. Examples of such materials, structures, and techniques are described in: H. F. Gray, Proc. 29th Intl. Field Emission Symp., p. 111, 1982; I. Brodie, IEEE Trans. on Electron Devices, 36, p. 2641, 1989; C. A Spindt, C. E. Holland, A. Rosengreen and I. Brodie, IEEE Trans. on Electron Devices, 38, p. 2355, 1991; E. A Adler, Z. Bardai, R. Forman, D. M. Goebel, R. T. Longo and M. Sokolich, IEEE Trans. on Electron Devices, 38, p. 2304, 1991; and M. Yuan, Q. Li, W. P. Kang, S. Tang and J. F. Xu, Journal of Vacuum Science Technology B, 12(2), p. 676-679, 1994. The most desirable properties for an electron emission cathode are low operating voltage, high emission current density and uniformity, and emission stability, longevity and reliability.
The unique material properties of diamond, including low electron affinity, wide band-gap, chemical stability, resistance to particle bombardment, hardness, and good thermal conductivity, are beneficial for vacuum microelectronics applications. However, due to the chemical inertness of diamond, the work reported in the prior art involves only planar diamond films, non-uniformly diamond coated silicon tips, or irregular ion-etched diamond conical structures. Control of the uniformity and microstructure of diamond film is essential for field emission device applications. Very high field emission current with diamond is achieved by proper design and configuration of a well structured diamond microtip emitter.
Although those skilled in the art have recognized that diamond has properties that make it potentially very useful as an emitter in microelectronic devices, that potential has remained unfulfilled up to now. Various emitter structures using diamond have been designed but their emission performance has been unsatisfactory. For example, many prior art diamond tipped emitter structures have been inefficient emitters or have produced emission currents that are unstable. To obtain high field emission efficiency in a solid state microstructure emitter, the tip of the emitter must be extremely sharp. In those few instances in the prior art where efficient and stable diamond tip emitters have been built, the fabrication techniques have been expensive and/or time consuming. Typically, the prior art to structures have been fabricated by a sputtering or deposition process that lays the diamond on a planar substrate. The resulting emitter structure must then go through extensive machining or other post-deposition shaping steps in an attempt to create a sharp tip that will perform adequately. In other prior art fabrication methods, additional steps must be taken to initiate diamond growth, such as by ion implantation of the substrate. For example, in U.S. Pat. No. 5,129,850, the inventors describe a method of fabricating an emitter having a diamond coating. Although the diamond coating may enhance the emission characteristics of the emitter (assuming that the device could actually be built as described), the device will not have the same desirable characteristics found in a solid, monolithic diamond emitter structure.
The fabrication and emission performance problems of the prior art have been overcome in the novel field emission devices and fabrication methods of this invention, using sharp tips of well patterned diamond microtip emitters (e.g., pyramid, knife edge, conical, volcanic cone, sharp pillar microstructures) for the development of vacuum field emitter element/arrays for vacuum microelectronics and sensor applications. The use of local electric field enhancement at sharp points, constructed by molding and micromachining techniques of diamond material as described here, utilizes plasma enhanced chemical vapor deposition (PECVD) to produce micron size or smaller structures, on a diamond film/field, with very sharp tip curvatures, such as less than 200 A. Several novel structures and devices are described, including a micro-patterned diamond emitter element/array, and related novel device structures in diode, triode, display, and sensor configurations.
To create the high performance diamond microtip structures of this invention, several novel fabrication steps are described, including deposition of diamond into cavities formed in a substrate mold, using a novel deposition process that preferably occurs in a sequence of smooth and standard deposition steps. In the smooth deposition step, small grain sizes are achieved at the tip of the emitter structure, with the standard deposition step producing larger grain sizes with an increased deposition rate.
The novel fabrication processes of this invention includes the ability to control the carbon graphite content of the diamond. This produces a diamond tip with an ideal or controllable balance of emission efficiency and durability. Emission performance of the structure is further enhanced by vacuum-thermal-electric treatment of the tips, hydrogen plasma tip sharpening, high temperature annealing, and application of thin metal coatings to the tips.
In another embodiment of the invention, diamond microtip emitters and emitter arrays are used as cathodes in novel diode and triode devices having integrated anode and/or grid structures.
High current emission from the patterned diamond microtip arrays was obtained at low electric fields. An emission current from the diamond microtips of 0.1 mA was observed for a field of  less than 10 V/xcexcm. Field emission for these diamond microtips exhibits significant enhancement in total emission current compared to silicon emitters. Moreover, field emission from patterned pyramidal polycrystalline diamond microtip emitter arrays, as fabricated by the inventors and described herein, is unique in that the applied field is found to be lower compared to that required for emission from Si, Ge, GaAs, and metal surfaces. The novel fabrication processes utilize selective deposition of diamond film in a cavity mold such as might be created in silicon, and subsequent creation of a free standing diamond substrate or plate with a diamond microtip emitter array. The processing techniques are compatible with IC and other micromachining fabrication technologies.